This invention relates to an apparatus for cutting and retrieving a porous membrane which is securely fastened to a membrane holder.
Porous membranes housed in secure fashion to rigid holders are widely used in many applications in which a sample solution is contacted with the surface of the membrane. Solutes of interest in the sample are captured on or bound to the surface of the membrane. Thereafter it is often desirable to remove the membrane from its holder and subsequently transport it to another location or device for further analysis.
In analytical techniques involving biomolecules, such as proteins or peptides, the researcher is often working with small quantities of expensive samples (particularly in terms of time and effort to produce and/or purify). Thus disruption of the membrane-containing sample must be kept to a minimum to avoid contamination which could skew the results of any subsequent analysis/processing of the sample or, worse yet, destroy the biomolecular sample under investigation.
The difficulties associated with the foregoing techniques become exacerbated when samples are filtered inside a centrifuge. In this instance, the sample is added to an elongated plastic container having a membrane securely fastened at the bottom of the container. Often the membrane is supported on its downstream side by an additional rigid plastic screen. Thus removal of the membrane from devices of this type has heretofore involved cutting away the plastic housing which holds the membrane with razor blades or clippers, a cumbersome and, in the case of razors, a potentially harmful alternative. Furthermore, once removed, the membrane must be handled either with forceps or other mechanical means to transport the cut membrane for subsequent analysis or processing. Additionally, it may be necessary to perform a secondary cut to trim the membrane such that only the active filter area is presented for analysis. Such manipulations can seriously degrade the sample to be analyzed.
The prior art includes numerous examples of cutters or punches used to separate a porous membrane from its holder device. For example, U.S. Pat. No. 5,146,794 describes a filter punch particularly useful for multi-well plates. While the punch described in the '794 patent represented a significant advance, membranes located at the base of multi-well plates are generally unsupported and easily accessible. In addition, this patent discloses a punch with a protruding member with a sharp point which pierces the center of the membrane. This impacts the useful filtration area of the membrane which can be a problem when dealing with small quantity biomolecule samples such as peptides.
An example of a centrifugal filter unit is the Prospin.TM. Sample Preparation Cartridge sold by Applied Biosystems, Inc. In this unit, a membrane is secured inside the bottom of an elongate container (termed an insert) without a rigid support on its downstream side. After filtration, the insert is inverted, fully exposing the membrane. A punch is placed on the downstream side of the membrane and pushed down to cut the membrane around the periphery of the insert causing it to separate from the insert. This technique can expose the membrane to contaminants and also requires that the membrane be retrieved with a secondary tool such as forceps.
Accordingly, it would be desirable to provide a device capable of cutting a membrane and retrieving the cut membrane in one operation with minimal chance of contaminating or otherwise compromising the sample to be analyzed. It would also be advantageous for such a device to function with both supported and unsupported membrane holder devices.